The present invention generally relates to a color-image processing method, and more particularly to a color-image processing method in which a two-color image such as a red-black image or a blue-black image can be formed in accordance with three color-component signals, such as color signals corresponding to red, green and blue, obtained by scanning a color original.
As a copy machine, a monochrome copy machine and a full-color copy machine are commercially available. The monochrome copy machine is relatively inexpensive, and thus it has become widely used. The full-color copy machine can produce a full-color picture having a gradation in brightness, but it is relatively expensive.
The full-color copy machine scans a color original to obtain three color-component signals. The three color-component signals correspond to colors R (red), G (green) and B (blue) having a gradation in brightness. The three color-component signals R, G and B are then processed so as to be converted into image signals corresponding to colors C (cyan), M (magenta) and Y (yellow). After that, the image signals C, M and Y are converted into four color-signals C, M, Y and BK (black). When a gradation is needed, a dot size for each color is changed.
However, color originals are not always a full color image having a fine gradation levels such as a color photograph. Many color-originals may be a picture or a document having additional letters in red or colored letters to enhance an importance of the letters. That is, many color-originals may be an image comprising only two colors, practically, such as a black color image added with a red or blue image.
Such two-color image can be copied, of course, using the full-color copy machine, however, there is a problem in that it must take a time to perform image forming operation for four colors. When an original has color portions for only enhancing it or distinguishing it from other portions, a chroma or brightness of the color portions in a copied print can be in a level in which the color of the color portion is not so important. In such a case, a color image formed by means of two-color image forming process is even better than that formed by means of a three-color image forming process, and the image processing time can be reduced to three quarters or one half of that required by the three-color image.
When forming the two-color image in accordance with three color image signal, there may occur a color shear due to a positional tolerance between three CCD lines. Such a color shear is hardly recognized in an image having continuously varied gradation level or hue.
However, the full-color copy machine generally uses a high resolution print engine so that a decrease in resolution is hardly recognized when a dot size is changed. Accordingly, when there is a slight color shear at an edge of a binary image having a high resolution without having a dot size change, a color may appear at an edge of a black image or black may appear at an edge of a color image.
Therefore, when forming a two-color image, in particular, two-color binary image, more care is to be taken regarding the color shear rather than that of the full-color image. Thus, a color shear correction is required to be performed on input color-image signals in addition to an accurate positioning of the three line CCDs.